FIG. 4 shows a schematic view of a background-art film driving apparatus (for example, see JP-A-8-217302). Examples of films may include a photosensitive film, an intermediate transfer film, a transfer film, etc. Here, a photosensitive film will be used by way of example, and a driving apparatus for driving the photosensitive film will be described.
In FIG. 4, the reference numeral 1 represents a film (photosensitive film); 2, a driving roller; 3, a skew correction roller; 4, a film support roller; and 5, a developing-unit opposed roller. The film 1 is supported by the driving roller 2, the skew correction roller 3, the film support roller 4 and the developing-unit opposed rollers 5 so as to form a path. The film 1 is rotated in the illustrated arrow direction by the driving roller 2.
The configuration of a skew correction mechanism portion will be described. The opposite ends of the skew correction roller 3 are retained by arms 8 and 8 respectively. The arms 8 and 8 are linked with brackets 9a and 9b including bearings through their shafts, respectively. The brackets 9a and 9b are linked with a not-shown frame body so as to retain the skew correction roller 3.
The movable bracket 9a can be rotated around a rotation axis a by a cam 10, while the arms 8 and 8 can be rotated around axes d through the bearings included in the brackets 9a and 9b respectively. Therefore, only one end portion of the skew correction roller 3 is moved in a circular trajectory with the rotation axis a as the center so as to correct the skew of the film 1.
Further, there is a structure in which a spring 20 is set between each arm 8, 8 and each bracket 9a, 9b so that the arm 8 can be moved to expand and contract axially. Thus, the skew correction roller 3 also has a function of applying tension to the film 1.
A process for producing an image on the film 1 will be described. As shown in FIG. 5, the film 1 is charged by a charger 15 and irradiated with a laser beam from a light source 16 so that an exposure point where a latent image should be produced on the film 1 is provided in a film path portion wound on the film support roller 4. Thus, a latent image is produced.
Successively, toner is attached to the latent image by a developing unit 17. The produced image is transferred onto paper, a transfer film or the like at a transfer point. After the transfer, charges of the film 1 are removed by a discharger 18, and residual toner on the film 1 is removed by a cleaner 19 in order to prepare next charging.
Here, as for the characteristic of the film 1, the circumferential length of the film 1 changes due to a change in temperature or humidity. When there arises a change in the circumferential length of the film 1 during a printing operation, the arms 8 moves to expand and contract due to the operation of the springs 20 in the skew correction mechanism portion so that the position of the skew correction roller 3 is changed, as shown by the broken-like portion in FIG. 5. In this manner, the change of the circumferential length is absorbed.
In the aforementioned background-art film driving apparatus, however, the film path changes near the skew correction roller 3 so that there arises a fluctuation in the film path length between the exposure point and the transfer point.
When there arises a fluctuation in the film path length between the exposure point and the transfer point as described above, the distance between the exposure point and the transfer point on the film 1 changes so that the position of the image formed in the exposure point is shifted from an original printing target point. Thus, there arises a problem of printing misalignment.
The movement trajectory of the movable end portion of the skew correction roller will be described. A trajectory not causing any change in the circumferential length of the film near the skew correction roller 3 is an elliptic trajectory b with focuses in the centers of the adjacent film support roller 4 and the adjacent developing-unit opposed roller 5, as shown in FIG. 6. According to a film skew correction system in the background art, the movement trajectory of the movable end portion of the skew correction roller is a circular trajectory c. Thus, a misalignment arises with respect to the elliptic trajectory b not causing any change in the circumferential length of the film. As a result, there is a problem that printing misalignment arises due to a change in the circumferential length of the film near the skew correction roller 3.